Hypothyroidism is a disorder of the thyroid gland – two butterfly-shaped lobes located in the neck just below the voice box. These glands are responsible for producing and secreting thyroid hormone (thyroxine), which affects nearly all body systems. Most significantly, the thyroid glands regulate your dog's metabolic rate. In hypothyroidism, not enough thyroxine is produced and that causes metabolism to slow.

Hypothyroidism has physical and behavioral effects on dogs. Physically, dogs with hypothyroidism tend to gain weight, may have bouts of diarrhea or constipation, and may suffer from various skin problems (e.g. dry, flaky skin and excess shedding). They may have an increased susceptibility to infections, have a lowered tolerance for cold, and may tire easily.

Dogs with classical hypothyroidism often seem lethargic and depressed. However, dogs with a "mild" or sub-clinical affliction may show a different set of behaviors. They may become anxious or fearful, become more aggressive, exhibit a compulsive disorder (e.g. excessive grooming or tail-chasing). Some dogs may also appear hyperactive and/or be slow learners.

Assessing whether hypothyroidism is contributing to (or causing) a dog's behavior problems is the first step.

How is an Assessment Performed?

If the pet is exhibiting any of the behaviors mentioned above, it should be evaluated for signs of hypothyroidism. For instance, if a dog has an anxiety disorder, and is showing two or more of the signs listed below, it may have mild (sub-clinical) hypothyroidism:

Year-round shedding

Dry, brittle coat

Slow regrowth of shaved areas of hair

Flaky skin

Increased susceptibility to infections

Allergy

Irregular heat cycles in intact female dogs

"Sad" facial expression

Worried expression

Hyperactivity and/or difficulty in learning

History of seizures

The Tests and Their Interpretation

Various tests are performed to confirm classical hypothyroidism. These tests include a complete blood count (CBC), biochemistry profile, urinalysis, thyroxine (T4) plasma level, TSH stimulation, and chest and abdominal X-rays, in certain cases. Some of these tests may be helpful when assessing sub-clinical hypothyroidism, but a blood sample is often all it takes.

Sub-clinical hypothyroidism may be diagnosed when:

1. A dog's T4 level is in the lower 25th percentile of the range even though the dog is young or middle aged, physically active, otherwise in good health, not receiving thyroid depressant medication, medication, and is not a sighthound (separate rules apply for the latter breeds). The level may be "within the normal range" but too low for normal function (i.e. is sub-optimal).

2. The dog is evidencing behavioral compatible with anxiety or aggression.

3. The dog is showing two or more mild physical signs of hypothyroidism (see list above).

Fortunately, subclinical and clinical hypothyroidism are easily treated by means of a twice-daily dose of synthetic thyroxine. During the course of therapy, the dog will need its serum thyroxine (T4) checked periodically to make sure the hormone level is appropriate. The first check should be performed 4-6 weeks after initiating therapy with thryoxine. The blood sample should be taken 4-6 hours after a dose of the medication to assess the peak level. The goal is to have levels of T4 boosted to the upper end of the normal range (or even slightly above). Behavioral improvement, if any, is assessed at the same time and a decision is made as to whether to a) adjust the dose (up or down) b) continue the treatment, c) dispense with treatment (levels optimized but no improvement). If treatment is successful, T4 therapy should be continued for the rest of the dog's life.